Initial Update: Wednesday, 27 January 2016 00:12 UTC
We identified an issue with ServiceHooks where notification will not fire. Impact is limited to TFS accounts in South Central US with ServiceHooks notification setup. The issue is already understood and a deployment is already on the way that would mitigate the issue.
We are working to resolve this issue and apologize for any inconvenience.
Sincerely,
VS Team Services Service Delivery Team